Thanks David but that doesn't really help.
For photos that I am still in the process of editing, I want the full size preview - I use the full screen view to help with filtering images and to check final edits etc. I wouldn't want to have an inferior preview for the full screen preview.
On the other hand, for older photos that are in my catalogue, but for which I have finished editing, I really don't need to keep the previews. I will only look at them through Lightroom occasionally, and I don't mind a small amount of lag for the preview to render on the fly on the odd occasion that I do.
I don't want to have to break apart my catalogue to separate old from new: that really diminishes the value of Lightroom's great library capabilities.
For the sake of managing SSD space, I will at some point move my older photos off to a NAS, but will still keep them in the catalogue. OK, I get the RAW file space back, but it seems that I'll never be able to reclaim any space in the ever growing previews cache.
This really seems like a significant weakness.